1. Technical Field
This invention relates to the art of roll-finishing planet pinion or sun gears with precise concentricity and accurate formation of gear teeth, particularly helical gear tooth elements, and to adapting such roll-finishing to gear blanks initially prepared with less concentricity.
2. Discussion of the Prior Art
Roll-finishing external helical gear teeth has displaced, in some plants, other finishing techniques of gear generating that depend upon metal cutting. An advantage is gained by use of roll-finishing dies to attain a much quieter gear at minimal expense; this is particularly attractive for fine pitch automotive gearing used in automatic transmissions.
U.S. Pat. No. 3,362,059 is a pioneering patent directed to roll-finishing of gears using roller dies with involute sides having differing pressure angles on opposite sides of the die teeth; such patent is commonly assigned to the assignee of this invention. The above patented process has heretofore required an arbor to accurately locate and position the gear blank, internally and axially, while undergoing roll-finishing. This is and remains a most sensitive aspect of the process because it not only demands that the arbor be positioned in a precise station lying in a precise relation to the rolling dies, but also that the internal openings of the gear blank and dies have concentric surfaces about their supporting axes to assure a high degree of accuracy of the gear teeth.
A significant future for the roll-finishing process exists, if it can be applied to gear blanks previously formed to near-net-shape without machining but accompanied by slightly less accurately formed surfaces. But how can conventional roll-finishing be carried out if the internal bore of the gear blank is slightly inaccurate, thereby frustrating internal chucking. For example, either conventional or hot-forged near-net-shape powder metal gear blanks will have a pressed internal bore pitch diameter concentricity accuracy of about 0.004 inches as opposed to an accuracy achievable with gear blanks finish-machined to about 0.001 inches or better. Thus, the internal bore of such powder metal gear blanks will not possess the extreme accuracy of gear blanks hobbed or machine-cut, but, of course, will not suffer from the expense and lower productivity associated with hobbing or machining.
Less accurately and nonconcentrically formed gear blanks (either as a result of using a nonmachining forming process or as a result of poor machining) cannot be located and positioned by arbors if gear tooth accuracy is to be achieved by roll-finishing. U.S. Pat. No. 3,599,463 recognized this problem and observed that in larger gears it was exaggerated; it disclosed a solution that included separate but matching guide surfaces on the dies as well as the gear blank. This solution would not be helpful when attempting to adapt the roll-finishing process to gear blanks formed by near-net-shape techniques and their accompanying reduced accuracy.